Low pressure melamine/veneer panel and method of making the same

ABSTRACT

A layered composite product for use as a floorboard, a door, a cabinet, a wall, etc. is disclosed. The product is a layered structure that includes a rigid substrate layer having two substantially planar sides, a resin-saturated sheet layer disposed on each of the substantially flat sides, and a veneer layer disposed on at least one of the sheet layers. Also provided is a method of making the aforementioned product, the method comprising the steps of forming the layered structure, and then compressing the structure in a pressing apparatus for a time sufficient to cure the resin at under heat and pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to pressure laminated wood products andmethods of making the same. More specifically, the invention relates tolow-pressure laminated wood products and a method of securing decorativeveneers to core substrates using resin-saturated sheets.

2. Brief Description of Related Technology

Layered, composite wood structures having an inner core substrate andveneer layers affixed to one or more major planer surfaces of the innercore substrate are well-known in the art. Such composite wood structuresexhibit the desirable characteristics of the veneer material whileproviding a cost savings over a solid structure made entirely of theveneer material. For example, a natural hardwood veneer, such as oak,may be secured to a less expensive substrate, such as high densityfiberboard, to produce a composite panel article. The composite woodstructures serve a variety of uses, such as floorboards, doors,cabinets, countertops, and walls, to name a few. Veneers used in formingthe composite wood structures are selected from a wide variety of woodmaterials depending upon economic considerations and the end-use of thestructure. Typical woods used as veneers in the past have included ash,birch, cherry, maple, oak, pine, poplar, and walnut.

Wood veneers may be secured to core substrates by lamination processesusing resin binders in the form of resin-coated papers. Commonly usedresins include thermosetting materials, such as melamine resins.Melamine resins, also known in the art as melamine/formaldehyde resins,typically exist in aqueous solution, although alcohols or other organicsmay also be used as solvents. A melamine-coated paper can be dried toremove water and/or solvent leaving a dry, solid state melamine-coatedpaper.

As described in Kirk-Othmer's Encyclopedia of Chemical Technology,Volume 14, pgs. 1074-91 (1995), a resin-coated paper may be placedbetween a veneer sheet and an inner core substrate to form a layered,composite wood structure. This layered structure is then heated in acompression molding press at a temperature of 130° C. (266° F.) to 150°C. (302° F.) for 20 minutes to 40 minutes. The initial heating causesthe resin to melt and flow into the voids and pores of the coresubstrate and the veneer sheet. With a sufficiently long press cycle,the resin polymerizes and crosslinks into an intractable networkstructure (a thermoset) bonding the core substrate to the veneer. Afterheating, the structure is cooled while in the press under pressure, andthereafter may undergo finishing operations, such as trimming and/orsanding, to form the final product.

Thermosetting materials, such as melamine, have been used due to theirsuperior resistance to discoloration when subjected to the conventionallamination temperatures and their exceptional bonding strength. Layeredwood composite structures made by a conventional compression moldingpress processes, as described above, however, may experience excessivewarpage toward the veneer side.

Wood veneer and substrate materials generally contain moisture. Changesin moisture content as a result of changes in atmospheric moistureconditions and wetting of the wood surface(s) by water generally tend tocause wood materials to warp or blister. The type of manufacturingprocess also may influence the moisture content of the formed woodproduct and thereby affect warpage. Thus, the original moisture contentof the veneer and substrate, as well as the amount of water added by theaforementioned binder, must be maintained low to avoid warp and/orblistering of the wood materials. During heat curing and/or pressing ofthe wood materials, water evaporates from the wood materials. Thisevaporation may impart stresses in the wood materials, causing them towarp. The evaporation of additional moisture from the binder used tosecure the various wood materials together may also contribute to woodwarpage.

Balancing veneer or heavy duty backer paper have been used in the pastto balance, or offset, the wood warping problem. In this case, where aveneer layer is placed on both faces of a substrate, any tendency of thewood materials to warp effectively cancel each other out. However, theuse of this additional veneer as well as board production isunnecessarily expensive where a second veneer serves no cosmeticpurpose. Furthermore, a second veneer is often not desired in certainapplications, such as store display boxes (e.g., jewelery displaycabinets).

Thus, it would be desirable to have a cost-efficient process for makinga laminated wood structure wherein the amount of warpage is drasticallyreduced, if not eliminated altogether.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more of the problemsdescribed above.

According to the invention, a method of making a laminated compositearticle includes the steps of (a) forming a layered structure comprisinga rigid substrate layer having two substantially planar sides, aresin-saturated paper layer disposed on each of the substantially planarsides, and a veneer layer disposed on at least one of the paper layers;and, (b) subjecting the structure to elevated temperature and pressurein a pressing apparatus for a press cycle time of about one minute orless.

The invention provides a laminated wood composite article comprising arigid substrate layer having two substantially flat sides, oneresin-saturated paper layer on-each of the substantially flat sides, anda veneer layer on at least one of the paper layers.

Other objects and advantages of the invention will be apparent to thoseskilled in the art from a review of the following detailed description,taken in conjunction with the drawing and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a partial, cross-sectional view of a laminatedcomposite wood article according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a method of making a low pressuremelamine/veneer panel is provided, the method generally includingforming a composite wood structure including a rigid substrate layerhaving two substantially planar sides, a resin saturated paper layerdisposed on each of the substantially planar sides of the substrate, anda veneer layer disposed on at least one of the paper layers. Thecomposite wood structure is subjected to elevated heat and pressure in apressing apparatus for a press cycle time of about one minute or less.

According to a preferred embodiment of the inventive method, there isinitially provided a substrate having a top planar surface and a bottomplanar surface. A surface of a resin-impregnated (or resin-saturated)low pressure laminate (hereafter "LPL") sheet is placed in contact withthe top surface of the substrate, and another surface of theresin-impregnated sheet is placed in contact with a surface of a woodveneer sheet. A second resin-impregnated sheet is placed in contact withthe bottom surface of the substrate, which is opposite (and parallel) tothe top surface of the substrate.

Referring to the drawing, a partial, cross-sectional view of a laminatedwood structure, generally designated 10, according to an embodiment ofthe invention is illustrated. The structure 10 includes a veneer sheet12, a first resin-impregnated LPL sheet 14, a core substrate 16, and asecond resin-impregnated LPL sheet 18. The structure 10 includes a coresubstrate 16 sandwiched between the first and second resin-impregnatedLPL sheets 14 and 18, respectively, and a veneer sheet 12 overlying thefirst resin-impregnated LPL sheet 14. The layered structure 10 islaminated under heat and pressure according to the method describedbelow to form a laminated composite wood article.

Suitable substrate materials for use in the laminated composite articlesof the invention include various wood materials, such as particleboardand low-, medium-, and high-density fiberboard. Preferably, however, thesubstrate is a medium-density fiberboard having a density of about 45pounds per cubic foot (lbs/ft³) and 65 lbs/ft³.

Generally, any sheet- or paper-like material can be used as the LPLlayers of the inventive composite, so long as the material can beimpregnated or saturated with a suitable resin composition. Suitablematerials may be prepared from cellulosic fibers, such as creped oruncreped kraft paper and the like, and alpha-cellulose sheet material.These sheets are typically regenerated cellulose paper, or similarfibrous material. It is preferable, however, to use an alpha cellulosesheet material. Furthermore it is preferred to use an alpha cellulosesheet having a basis weight in the range of about 40 pounds per ream toabout 100 pounds per ream (about 18 kilograms per ream to about 45kilograms per ream).

The resin of the resin-impregnated LPL sheet comprises a melamine resin,and preferably comprises either (a) a melamine/urea blend and additives,or (b) about 98 wt. % melamine with trace concentrations of additives. Apreferred melamine/urea blend includes one comprising about 60 wt. %melamine and about 40 wt. % urea based on the total weight of the resincomposition. However, other blends of melamine and urea also can beused. Additives comprising the resin composition include one or morecatalysts to control resin rate of cure, wetting agents, internalrelease agents, and plasticizers. Typically, the total concentration ofadditives comprising the resin composition is about 2 wt. % or lessbased on the total weight of the resin composition.

The impregnation of the LPL sheet generally is achieved by transformingthe resin composition into a solution with a suitable organic solventand/or water, and by applying the resultant solution to the sheet or byimmersing the sheet into the resultant solution. In order to minimizeformaldehyde emissions in subsequent heating and drying steps, it ispreferable to saturate (as opposed to simply coat) the sheet with theresultant solution. After the sheet has been saturated with theresultant solution, the sheet is preferably dried at room temperature orunder application of heat to effect the expulsion of the volatileorganic component and/or water to the fullest extent possible. Somepolymerization occurs during this volatilization step. Suitable ovensthat can be used to accomplish this drying step are air flotation ovens.

By drying the sheet by the foregoing procedure a resin-impregnated (alsoreferred hereafter as a "resin-saturated") sheet is formed, whichcontains the resin in a dry, solid state. The concentration of resin inthe resin-impregnated sheet is preferably in the range of about 50 wt. %to about 65 wt. % based on the total weight of the resin-impregnatedsheet. The pH of the resin saturating the LPL sheet is about 8.0 toabout 9.5, and preferably is about 8.5 to about 9.0. The moisturecontent of the resin in the sheet is preferably about 6 wt. % to about 8wt. % based on the weight of the resin-impregnated sheet, and morepreferably is about 7 wt. % based on the weight of the resin-impregnatedsheet.

Any wood-like material is suitable as a veneer material for useaccording to the invention. Such materials include softwood materials,man-made wood veneers, and hardwood materials, wood composite materials,and high pressure laminates, such as a composition of a phenolicresin-impregnated kraft sheet covered by a melamine decorative sheet.Suitable wood composite materials include softboard, medium densityfiberboard, hardboard, particleboard, plywood, and waferboard. Themoisture content of the veneer materials is preferably in the range ofabout 7 wt. % to about 10 wt. % based on the weight of the veneer, andmore preferably should be between about 8 wt. % to about 9 wt. % basedon the weight of the veneer.

The layered structure is subjected to heat and pressure in a pressingapparatus including a first platen and a second platen. Each platen,optionally, may contain caul plates which are often used to impart apattern to the wood or resin-impregnated sheet. Furthermore, the pressedstructure also has a reduced tendency to adhere to the caul platescompared to the platens.

The layered structure is arranged in the pressing apparatus such thatduring pressing the first platen is in thermal contact with the veneerlayer. The first platen preferably has a temperature of about 350° F.(177° C.) to about 405° F. (207° C.), and preferably has a temperatureof about 375° F. (190° C.) to about 385° F. (196° C.). The second platenpreferably has a temperature of about 320° F. (160° C.) to about 350° F.(177° C.), and preferably has a temperature of about 335° F. (168° C.)to about 340° F. (171° C.). The first and second platens preferablycompress the layered wood structure at a pressure of about 325 psi toabout 425 psi, more preferably about 350 psi to about 380 psi, for apressing period sufficient to cure the resin at the aforementionedtemperatures and pressures. Preferably, the pressing period is about 60seconds or less, more preferably about 50 seconds or less, and mostpreferably about 30 seconds to about 40 seconds. The foregoing presscycle conditions of time, temperature, and pressure, allow the resin tosufficiently cure and form strong bonds between (a) the LPL sheet andthe substrate and (b) between the LPL sheet and the veneer.

An important feature of the invention is the simultaneous lamination ofthe resin-impregnated LPL sheets to both top and bottom surfaces of thesubstrate material. This simultaneous lamination ensures minimal warpageof the substrate material and further ensures minimal water penetrationinto the substrate. The use of a resin-impregnated LPL sheets accordingto the invention also substantially prevents the evaporation of waterfrom the wood itself, thereby minimizing wood warpage.

It has been found that the rate of resin cure also serves to control theoccurrence of wood warpage. Thus, press temperatures and catalystconcentrations in the resin help fine-tune the elimination of woodwarpage. For example, the higher the press temperature, the higher thetendency of the wood to warp. Therefore, press temperatures (for each ofthe first platen and second platen) are carefully selected to accountfor any tendencies of the wood materials to experience warpage based onthe catalyst concentration. The tendency of the wood to warp toward theveneer side is offset by the catalyst concentration of the resin in thebottom LPL sheet as well as the temperature of the platen in thermalcontact with the LPL sheet.

EXAMPLE

A 1/32 inch (0.079 cm) maple veneer having a moisture content of about 7wt. % was laminated to a first major planar surface an 11/16 inch (1.75cm) thick industrial grade particleboard. One 67 pounds per ream (30.4kilograms per ream) resin-saturated white paper (alpha cellulose),having a resin content of about 62 wt. % resin content and about 7.5 wt.% organics content, was placed between the veneer and the first majorplanar surface of the particleboard. A second, equivalentresin-saturated white paper was place on a second major planar surfaceof the particleboard. Thus, the layered wood structure has aconfiguration as shown in the sole drawing figure.

The layered wood structure is placed in a single-opening, hot oil heatedpressing apparatus having a first (top) platen, a caul plate affixed tothe first platen, and a second (bottom) platen, wherein the caul plateof the first platen is in thermal contact with the veneer layer, and thesecond platten is in thermal contact with the second resin-saturatedwhite paper. The apparatus subjected the layered wood structure heat andpressure. The first (top) platen (and caul plate) was maintained at atemperature of about 350° F., the second (bottom) platen was maintainedat a temperature of about 320° F., at a pressure of about 380 psi for apress cycle time of about 55 seconds. The resultant panels wererelatively flat and passed an ANSI Hardwood Plywood & Veneer Association(HPVA) Type 1 glue bond line test.

The foregoing detailed description is provided for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications within the scope of the invention will beapparent to persons of ordinary skill in the art.

I claim:
 1. A method of producing a laminated composite article, saidmethod consisting essentially of the steps of:(a) forming a layeredstructure comprising a rigid substrate layer having two substantiallyflat sides, a resin-saturated sheet layer disposed on each of saidsides, and a veneer layer disposed on only one of said sheet layers;and, (b) subjecting said structure to elevated temperature and pressurein a pressing apparatus for a time sufficient to cure said resin at saidtemperature and pressure.
 2. The method of claim 1, wherein saidpressing apparatus comprises a first platen and a second platen, saidfirst platen having a temperature of about 350° F. to about 405° F., andsaid second platen having a temperature of about 320° F. to about 350°F.
 3. The method of claim 1, wherein said pressing apparatus subjectsthe structure of step (a) to a pressure of about 325 psi to about 425psi.
 4. The method of claim 1, wherein said time is about one minute orless.
 5. The method of claim 1, wherein said time is about 30 seconds toabout 40 seconds.
 6. The method of claim 1, wherein said resin of saidresin-saturated sheet is a material comprising melamine.
 7. The methodof claim 6, wherein said resin of said resin-saturated sheet is amaterial comprising about 98 wt. % melamine.
 8. The method of claim 6,wherein said resin of said resin-saturated sheet is a materialcomprising a melamine/urea blend.
 9. The method of claim 8, wherein saidresin of said resin-saturated sheet is a material comprising about 60wt. % of melamine and about 40 wt. % of urea.
 10. The method of claim 6,wherein said resin comprises about 45 wt. % to about 65 wt. % of theresin-saturated sheet.
 11. The method of claim 1, wherein said sheet ofsaid resin-saturated sheet is an alpha cellulose sheet having a basisweight of about 40 pounds per ream to about 100 pounds per ream.
 12. Themethod of claim 1, wherein said rigid substrate layer is a materialselected from the group consisting of particleboard, low-densityfiberboard, medium-density fiberboard, and high-density fiberboard. 13.The method of claim 1, wherein said veneer layer comprises a woodmaterial having a moisture content of about 7 wt. % to about 10 wt. %based on the weight of the veneer.
 14. A method of producing a compositearticle, comprising the steps of:(a) providing a rigid substrate layerhaving two substantially flat sides; (b) providing, on each of saidsides of the substrate layer, a sheet layer that is substantiallysaturated with a thermosetting resin; (c) providing, on only one of saidsheet layers, a veneer layer to form a lavered structure consistingessentially of said substrate layer, said sheet layers and said veneerlayer; (d) providing the layered structure of step (c) in a pressingapparatus; and (e) subjecting the layered structure of step (d) toelevated temperature and pressure in said pressing apparatus for aperiod sufficient to cure said resin at said temperature and pressure.15. The method of claim 14, wherein:the product of step (c) is providedin a pressing apparatus including a first platen and a second platen;and, in step (e) said first platen has an elevated temperature in therange of about 350° F. to about 405° F., and said second platen has anelevated temperature in the range of about 320° F. to about 350° F. 16.A method of producing a laminated composite article, said methodcomprising the steps of:(a) forming a layered structure comprising arigid substrate layer having two substantially flat sides, aresin-saturated sheet layer disposed on each of said sides, and a veneerlayer disposed on at least one of said sheet layers; and, (b) subjectingsaid structure to elevated temperature and pressure in a pressingapparatus for a time sufficient to cure said resin at said temperatureand pressure, said pressing apparatus comprising a first platen and asecond platen, said first platen operating at a temperature differentfrom an operating temperature of said second platen.
 17. The method ofclaim 16, wherein said operating temperature of said first platen is ina range of about 350° F. to about 405° F., and said operatingtemperature of said second platen is in a range of about 320° F. toabout 350° F.
 18. The method of claim 16, wherein said pressingapparatus subjects the structure of step (a) to a pressure of about 325psi to about 425 psi.
 19. The method of claim 16, wherein said time isabout one minute or less.
 20. The method of claim 16, wherein said timeis about 30 seconds to about 40 seconds.
 21. The method of claim 16,wherein said resin of said resin-saturated sheet is a materialcomprising melamine.
 22. The method of claim 21, wherein said resin ofsaid resin-saturated sheet is a material comprising about 98 wt. %melamine.
 23. The method of claim 21, wherein said resin of saidresin-saturated sheet is a material comprising a melamine/urea blend.24. The method of claim 23, wherein said resin of said resin-saturatedsheet is a material comprising about 60 wt. % of melamine and about 40wt. % of urea.
 25. The method of claim 21, wherein said resin comprisesabout 45 wt. % to about 65 wt. % of the resin-saturated sheet.
 26. Themethod of claim 16, wherein said sheet of said resin-saturated sheet isan alpha cellulose sheet having a basis weight of about 40 pounds perream to about 100 pounds per ream.
 27. The method of claim 16, whereinsaid rigid substrate layer is a material selected from the groupconsisting of particleboard, low-density fiberboard, medium-densityfiberboard, and high-density fiberboard.
 28. The method of claim 16,wherein said veneer layer comprises a wood material having a moisturecontent of about 7 wt. % to about 10 wt. % based on the weight of theveneer.
 29. The method of claim 16, wherein said veneer layer isdisposed on only one of said sheet layers.
 30. A method of producing acomposite article, comprising the steps of:(a) providing a rigidsubstrate layer having two substantially flat sides; (b) providing, oneach of said sides of the substrate layer, a sheet layer that issubstantially saturated with a thermosetting resin; (c) providing, on atleast one of said sheet layers, a veneer layer; (d) providing theproduct of step (c) in a pressing apparatus; and (e) subjecting theproduct of step (d) to elevated heat and pressure in said pressingapparatus for a period sufficient to cure said resin at said temperatureand pressure, said pressing apparatus comprising a first platen and asecond platen, said first platen operating at a temperature differentfrom an operating temperature of said second platen.
 31. The method ofclaim 30, wherein:in step (e) said operating temperature of said firstplaten is in a range of about 350° F. to about 405° F., and saidoperating temperature of said second platen is in a range of about 320°F. to about 350° F.